1. Field of the Invention
The present invention relates to a photoelectric conversion apparatus and, more particularly, to a photoelectric conversion apparatus formed by using a large-area process, which is suitably used as a two-dimensional photoelectric conversion apparatus for performing a one-to-one read operation in, e.g., a facsimile apparatus, a digital copying machine, or an X-ray image pickup or imaging apparatus.
2. Related Background Art
Conventionally, as a document reader in a facsimile apparatus, a copying machine, an X-ray image pickup apparatus, or the like, a read system using a reducing optical system and a CCD type sensor has been used. With the recent development in photoelectric conversion semiconductor materials typified by hydrogenated amorphous silicon (to be referred to as a--Si hereinafter), there has been a remarkable development in a so-called contact type sensor, which is obtained by forming photoelectric conversion elements and a signal processing unit on a substrate having a large area, and designed to read an image of an information source through a one-to-one optical system. Since a--Si can be used not only as a photoelectric conversion material but also as a thin-film field effect transistor (to be referred to as a TFT hereinafter), a photoelectric conversion semiconductor layer and a TFT semiconductor layer can be formed at the same time.
Furthermore, switching elements such as a thin-film field effect transistor and a capacitive element exhibit good matching and have the same film structure. For this reason, these elements can be formed as a common film at the same time. In addition, a photoelectric conversion apparatus having a higher S/N ratio can be manufactured at a lower cost. Furthermore, since each capacitor has an insulating layer commonly used as an intermediate layer and can be formed to have good characteristics, a high-performance photoelectric conversion apparatus capable of outputting the integral value of pieces of optical information obtained by a plurality of photoelectric conversion elements with a simple structure can be provided. With the use of a low-cost, large-area, high-performance photoelectric conversion apparatus, a facsimile apparatus or X-ray apparatus with added values can be provided.
In manufacturing a photoelectric conversion apparatus having a large screen, it is difficult to completely remove minute dust in the manufacturing process, especially dust peeling off from the wall of a thin-film deposition apparatus in the process of depositing an amorphous silicon layer on a substrate, and dust left on a substrate when a metal layer is deposited thereon. For this reason, with an increase in the size of a substrate, it becomes more difficult to eliminate wiring (interconnect) failures caused by dust and the like, such as a short circuit and an open circuit (disconnection) or wiring layers. In manufacturing a large-screen photoelectric conversion apparatus 121 by using one substrate, as shown in FIG. 1, with an increase in the size of a substrate, the yield per substrate decreases, and at the same time, the lost revenues per substrate increase.
As described above, at present, it is difficult to sufficiently decrease the cost of a large-area photoelectric conversion apparatus using one large-area substrate. Under the circumstances, a large-area photoelectric conversion apparatus is manufactured by using a plurality of substrates, e.g., silicon wafers or thin glass plates, having photoelectric conversion elements formed on their surfaces, and mounting the substrates on a base in the form of an array.
FIG. 2A is a schematic plan view showing such a photoelectric conversion apparatus obtained by two-dimensionally arranging a plurality of substrates. FIG. 2B is a schematic side view of the apparatus. Referring to FIGS. 2A and 2B, the apparatus includes substrates 1 and a base 2. An adhesive 51 is used to fix the substrates 1 to the base 2.
As the substrates 1, silicon wafers or thin glass plates are generally used. The thickness tolerance of silicon wafers is .+-.25 .mu.. The thickness tolerance of thin glass plates is .+-.200 .mu.. In general, semiconductor elements are formed on the upper surfaces of the substrates 1. In mounting the semiconductor elements on the substrates 1, the lower surfaces of the substrates 1 are bonded to the base 2 in most cases.
When, however, the substrates 1 are fixed to the base 2 with the adhesive 51 with a constant thickness, the respective substrates 1 exhibit level gaps, resulting in considerable deterioration in the performance of the photoelectric conversion apparatus.
FIG. 2B is a schematic side view showing such a level gap B between the substrates. For the sake of easy understanding, the gap B is emphasized. As shown in FIG. 2B, thicknesses T.sub.1 and T.sub.2 of the substrates 1 exhibit a variation. If, therefore, a thickness t.sub.2 of the adhesive 51 is made constant, the upper surfaces (semiconductor element portions) of the respective substrates 1 are set at different levels, resulting in the level gap B between the substrates 1.
With the occurrence of the level gap B between the substrates, the distance between a phosphor formed on a given substrate, or an object (original), and a semiconductor element may increase beyond an allowable range. As a result, the object may become out of focus to cause a decrease in resolution or sensitivity.
When a phosphor is to be bonded to the semiconductor element surfaces of the substrates 1, since the semiconductor element surfaces of the substrates 1 in the array have different levels, it is impossible to tightly bond the phosphor to all the substrates 1. Even if, the phosphor can be bonded to the substrates 1, such level differences may cause the phosphor to peel off.
Conventionally, in order to eliminate the level gap B and set the respective substrates at the same level, the substrates are polished to the same thickness in advance. Such a process, however, requires much time and many steps, resulting in an increase in cost.